Arterial blood oxygenation improved repeatedly after sedation and paralysis in a 27-year-old woman requiring mechanical ventilation for the adult respiratory distress syndrome. Oxygen consumption and cardiac output decreased proportionately after paralysis so that the partial pressure of oxygen in mixed venous blood remained unchanged. Paralysis eliminated inspiratory distortion of the airway pressure waveform and prevented forceful use of expiratory musculature. A flow-related reduction of venous admixture or recruitment of lung volume may best explain the beneficial effect of muscle relaxation on arterial saturation.